1. Field of the Invention
This invention relates to a system used for monitoring the transmission capability of electrical power transmission lines. The transmission capability of power lines will vary with the temperature of the line because the current carrying capacity is limited by the allowable sag of the line between transmission towers. In order to adequately control the load on a line it is necessary to know the condition of the line in terms of the sags of its spans. The present invention provides a system for monitoring such overhead transmission lines so that the load on the line may be adequately and properly controlled.
2. Description of the Prior Art
A major problem in overhead power transmission lines is that of clearance between the line and the nearest point on earth. If the line becomes too close to the earth or adjacent structures, problems can arise from electric flashover from the power line which can cause extensive electrical damage. For this and other reasons it is necessary to limit the current of these transmission lines so that any undue sag may be prevented. The major cause of increased sag is that of heating of the line. As the temperature increases the power line expands and clearance problems may arise.
The heating of the line results from various sources. One cause is the heat that is generated in the conductor by the electrical current flowing through it which causes I.sup.2 R losses in the conductor. Thus as the current in the line increases there is a greater generation of heat with the resulting increase in line sag. Line heating is also effected by solar heat and ambient temperature in the surrounding area. This heating of the line is offset by the cooling effect of wind passing over the line and heat radiated from the conductor.
Traditionally, transmission lines were rated based on an assumed combination of worst cooling conditions, consisting of a combination of expected highest ambient temperature, solar radiation and a low wind speed. Such traditional current ratings were highly conservative. To take adavantage of this conservatism, methods have been developed to either monitor some of the cooling conditions or the actual temperature of the conductor and to adjust current ratings based on such monitored data.
In the prior art, a number of methods have been used for determining line temperature. A first such method is that of theoretical calculation. Assumptions are made of wind speed and direction, ambient temperature and solar radiation and calculations are made for arriving at the line condition. Because the calculations are based on theoretical assumptions, the result can be at considerable variance from the actual line condition which might permit greater line current than exists or on the other hand dictate a lower actual line current.
In some instances weather stations have been established in the general location of the transmission line in order to monitor the weather to thus provide somewhat more reliable data that is then used to calculate the line conditions including the temperature of the line. A third method for monitoring the line known to the prior art is to provide sensor devices mounted on the conductor along the length of the line at various intervals to measure conductor temperatures from which load capacity can be determined. These various systems of the prior art are disclosed for example in U.S. Pat. Nos. 4,268,818 and 4,420,752 and 4,806,855. These later monitors have been somewhat more effective in identifying actual temperatures of the conductors. However one drawback of such systems has been that such sensors provide a measurement only at one point on the line and large number of sensors are thus required to cover a long span or series of spans because temperatures will vary considerably along the length of a span. Further they require extensive special communications methods such as FCC site specific radio licenses. Furthermore, because the sensor modules are mounted on the energized conductor, the manufacturing and installation cost of the sensors is complicated and expensive.
A further disadvantage of the prior, conductor temperature based, rating methods is that they cannot take into account creep, which is progressive stretching of conductor, caused by variation of conductor loading. The design sag and tension tables of conductors, such as the one shown in Table 1, determine the conductor tension and sag in initial condition (before any creep) and final condition (after calculated maximum creep). As shown in Table 1, the resulting uncertainty between the sags can be more than 10% of sag, and equivalent to a temperature uncertainty of 25 to 30 degrees C. This uncertainty is eliminateed if the lines are rated based on conductor tension.